Instrumentation: Laser Interferometry Space Antenna (LISA)

LISA and Gravitational Waves.

The LISA (Laser Interferometry Space Antenna) mission will open up a new window on the universe by enabling the observation of low frequency gravitational waves. It can be expected to have an impact as significant as the development of infra-red or X-ray astronomy. LISA is a joint ESA-NASA mission that involves having three spacecraft flying approximately 5 million kilometres apart in an equilateral triangle formation. Together, they act as a Michelson interferometer to measure the distortion of space caused by passing gravitational waves.

The mission goal of LISA is to detect and observe gravitational waves from massive black holes and galactic binaries with periods in the range of a few seconds to a few hours (i.e. 10E-4 to 10E-1 Hz). Ground based interferometers, such as LIGO, find this range inaccessible due to the background of local gravitational noise arising from atmospheric effects and seismic activity. Ground-based interferometers are also physically limited in length to a few kilometres, restricting their coverage to events such as supernova core collapses and binary neutron star mergers.

The development of drag-free technology is central to the exploitation of space as an environment for missions in Fundamental Physics. Missions such as Gravity Probe B, LISA and STEP are key space missions that exploit drag-free technology to achieve their scientific goals.

Drag-Free Control System

Three identical spacecraft will orbit the sun at a distance of 1 AU, trailing the Earth by 20 degrees. Proof Masses within each spacecraft are
shielded from all external EM radiation, so that their motion describes a perfect geodesic. These serve as optical references to the 1 Watt 1064nm
laser used to measure their relative positions in space. In such missions, the key is to ensure that the proof-mass (not the surrounding spacecraft)
remains in inertial space within the bandwidth required to achieve the science goal. This consideration leads naturally to a drag-free control system where
the spacecraft is decoupled as much as possible from the surrounding spacecraft.

Drag-Free Control Loop.

Capacitive sensor techniques are at present widely adopted for proof-mass position sensing. However, this solution is intrinsically unstable, since they can apply substantial forces to the proof mass and require conducting surfaces in close proximity to it. Interferometric optical sensors are a significant improvement over capacitive sensing, since they provide essentially zero-stiffness, high sensitivity and can operate over a large range (i.e. large separations between spacecraft structure & the proof-mass).

Furthermore, the inherent stability and accuracy offered by optical interferometers could provide drag-free operation at ultra-low frequencies. At Birmingham we have been developing an interferometric sensor, which may be the basis for a drag-free sensor.